Toughness is one of the characteristics to guarantee quality of a tool steel product. Toughness is the strength of a tool steel product against brittle fracture.
Brittleness breakage which has been known includes notch brittleness, temper brittleness, cold brittle fracture, etc. In order to guarantee the safety of a tool steel product, it is necessary to evaluate toughness of the product against them.
Parameters to evaluate the toughness include Charpy impact value (kg.m/cm.sup.2), Izot impact value (ft.lb), absorbed energy, (upper) shelf energy, brittleness transition temperature (energy transition temperature, fracture transition temperature, 15ft-lb transition temperature, etc.), fracture toughness (plane strain fracture toughness value, energy release rate, elasticplastic fracture toughness value), etc.
In recent times hot metal molds used for hot working of metal material or the like, are getting larger and the environment in which they are used is getting severe. Under such circumstances, quality assurance of metal molds (hardness and toughness) after heat treatment has become significant. Specifically, if hardness and toughness of metal molds can be precisely evaluated non-destructively, it becomes possible to obtain both properties which conflict with each other, in optimum combinations, and quality assurance and long service life of metal molds can be achieved. At present, however, even though hardness is measured, there is no way available to non-destructively evaluate toughness. Therefore, insufficiency of toughness due to lowering of hardening and cooling velocity with enlargement of the metal mold cannot be accurately determined. This sometimes results in a large crack being formed at the beginning of use.
The present inventor has paid attention to Barkhausen noise (hereinafter called as "BHN") signals which can non-destructively detect any changes in composition sensitively because toughness of a metal mold is closely related to thermally treated composition Applicant also studied the relation among BHN signals standard value (hereinafter called as "dVp") of a square sum of output voltage of total BHN signals produced in the process of magnetization (hereinafter called as "Vp"), tempering hardness (hereinafter called as "It") and Charpy impact value (hereinafter called as "Ch") using a hardened and tempered material from standard hardening temperature (1020 .degree. C) of 0.4C-SCr-Mo-V hot metal mold steel, and reported a method to non-destructively evaluate toughness (Ch) of subject steel from a relational expression as Ch=f(dVp,H) ("Iron and Steel", 1990 (1989), Vol. 5, P. 833). According to this report, Ch is estimated by the following formula: EQU Ch=.delta..sub.1 +.gamma..sub.1.H(kg.m/cm.sup.2) EQU .delta..sub.1 =a.sub.1 +a.sub.2.log(a.sub.3 +a.sub.4.dVp) EQU .gamma..sub.1 =b.sub.1 +b.sub.2.log(a.sub.3 +a.sub.4.dVp) EQU a.sub.3 =c.sub.1 +c.sub.2.H+c.sub.3.H.sup.2 EQU a.sub.4 =d.sub.1 +d.sub.2.H+k.sub.3.H.sup.2 EQU HRC43.ltoreq.H.ltoreq.HRC51 hardening temperature: 1020.degree. C.
Ch of a tool steel which has been hardened and tempered from a hardening temperature of 1020.degree. C. can be non-destructively evaluated from the above formula.
But, the actual hardening temperature of a metal mold is generally set at a level up to about 1015.degree. C. when toughness is given priority and at a level up to about 1035.degree. C. when strength is given priority. When the hardening temperature varies, the microstructure of a metal mold is different even when the tempering hardness (H) is same. Since toughness (Ch) is sensitive to microstructure, it is assumed that toughness (Ch) is different when the hardening temperature is different.
Therefore, when the hardening temperature is different from 1020.degree. C., toughness (Ch) cannot be accurately assumed by using the relational expression of Ch-dVp-H, obtained from the aforementioned report of the Inventor. In order to assume Ch correctly, it is necessary to independently study the relation of Ch-dVp-H at various hardening temperatures.